1. Field of the Invention
An improved method and apparatus used in the manufacture of an LED (light-emitting diode) printhead or the like is described, and more particularly the invention is directed to an improved method and apparatus for assembling LED dice or other recording elements to a tile module to provide improved Z-axis dimensional control.
2. Brief Description of the Prior Art
In the prior art as described in U.S. Pat. No. 4,942,405 LED dice and silicon driver chips therefor are assembled upon a metallic tile to form a testable module. After testing of the module, the modules are assembled upon a motherboard to form an LED printhead having a single row of several thousand LED's that may be used for recording as a non-impact printer. Typically, a lens such as a Selfoc lens (trademark of Nippon Glass Co. Ltd.) will be mounted to the printhead to focus light from the LED's onto a recording photoreceptor such as a photoconductive drum or web or other photosensitive surface such as photographic film.
As noted in the referenced patent, precision tolerances must be maintained in assembling the printhead and in forming the various components. In describing the invention disclosed herein, nomenclature set forth in the referenced patent will be employed. Thus, the face of the LED die on which the LED's are formed is referred to as the front and the opposite face as the back. A similar definition is used for the other parts of the assembly such as integrated circuit driver chips, mounting tiles and the like except that in each case, the face facing in the same direction as the LED's is referred to as the front.
Thus, a coordinate system is defined as having an x-direction along the line of LED's. A y-direction is defined as being in the plane of the LED's perpendicular to the x-direction. The z-direction is defined as being normal to this plane and is the direction in which the light output from the LED's is generally directed and may be thought of as the height direction of the LED's. Z-axis height control is important since the typical Selfoc lenses used have an associated depth of field, and uniform z-height control is required in order to stay within the field so as to minimize the pixel size variation of the transmitted image.
In the embodiments described for manufacturing of LED printheads disclosed in the referenced patent, there are basically described two approaches for Z-axis dimensional control. In a first approach, three LED dice are assembled onto a tile in a sub-assembly fixture where x and y-dimensional control is maintained but no z-axis control is provided. Thereafter, the tile with LED dice and driver chips adhesively attached thereto are mounted to a motherboard and z-axis control provided by supporting the front faces of all the tiles upon rails so that these front faces are accurately located while adhesive between the motherboard and the backs of the tiles accommodates the differences in dimension in the z-axis direction between the various tiles. A problem with this approach is that z-axis control of the gallium arsenide LED's is indefinite since the only known surface is the front face of the tile. A further problem with this approach is that a portion of the front of the tile must be left clear up to at least the assembly step of mounting the tiles to the motherboard. In addition to supporting the LED dice and driver chips for driving the LED's, the tile may also support a printed circuit board or spreader board for distributing electrical signals along the length of the printhead to the driver chips and wiring for connecting the various components.
In a second approach described in this patent, a sub-assembly fixture for mounting LED dice and driver chips to a tile employs recesses or pockets into which the LED dice and driver chips are placed. The front face of the LED dice engage a bottom surface of the pocket which is accurately spaced from the rails or reference faces formed on the sub-assembly fixture. A screen-printed adhesive is coated on the tile and the tile is supported on the rails over the LED dice and driver chips for adhesive bonding. Thus, the adhesive between the tile and LED dice accommodates variations in height of the components in the subassembly fixture. This approach still retains the problem of using some space on the front face of each tile during forming of the modules. Presumably, the modules will be assembled on the motherboard using the front face of the tile as a reference or datum and therefore clearance must be provided on the front face of the tile during this later assembly operation as well. This is undesirable since space on the tile is limited due to the area thereof occupied by the various components.